In recent years, 110 size cameras have come into wide use and it has been desired that prints obtained by enlarging pictures taken on such small sized picture planes have a quality nearly equal to those attained by enlarging large sized picture planes. Therefore, it has been necessary to develop color photographic materials having excellent granularity and sharpness even when enlarged to high magnification.
In general, visual sharpness is deteriorated with an increase in enlargement magnification. Sharpness may be evaluated based on the MTF (Modulation Transfer Function) curve in the case of high enlargement magnification (see T. H. James, et al., The Theory of the Photographic Process, 3rd ed., p. 536, Macmillan, New York (1966)). For instance, in the case that a picture plane is enlarged in size with a magnification 1.7 times as high as in the case to be a reference standard. If an MTF value equal to that at the spatial frequency increased by a factor of 1.7 is obtained in such a case, the resulting print would be theoretically expected, as illustrated in FIG. 1 to have a sharpness equivalent to that obtained from a negative image plane whose size is 1.7 times the size employed in the above-described case, provided that its sharpness is compared with that of the reference standard using prints having the same size. Thus, if a sharpness in a high spatial frequency region is heightened, deterioration of visual sharpness due to enlargement can be prevented from occurring even in the case of high magnifications.
In order to heighten the sharpness in the high spatial frequency region, it is necessary to raise the MTF values in the high spatial frequency region. The heightening of the sharpness in the high spatial frequency region can be attained by suppressing the occurrence of the light-scattering phenomenon as much as possible. For this purpose, it is essential to reduce the thickness of emulsion layers each (to about 1/2), that is, to render the photographic film thin.
As for measures for thinning a photographic film, there are reducing the amounts of the silver halides coated, reducing the content of gelatin used as a binder, reducing the amounts of couplers coated, reducing the amounts of high boiling point solvents used for dispersion of the couplers, and so on.
Among these measures, reduction of the coated silver amount can have the greatest effect upon suppression of light-scattering, but it causes a deterioration of granularity. On the other hand, a reduction of the gelatin content causes changes in the pressure characteristics of the emulsions and a reduction of coated coupler amounts causes a decrease in color densities and other undesirable side effects. Accordingly, it is important to reduce the amounts of organic solvents to use of dispersing couplers.
However, in the conventionally used method of dispersing color couplers in high boiling point solvents, it is difficult to reduce the amounts of the solvents beyond a certain limit from the viewpoints of solubility, coated film stability, and so on.
Accordingly, various investigations have been undertaken, and it has now been found that if so-called polymer couplers are employed, the amounts of high boiling point solvent(s) can be reduced to a great extent without the above-described side effects. As for the polymer couplers, a brief description is given in T. H. James, et al., The Theory of the Photographic Process, 4th ed., p. 347. Also, as for the MTF, there are descriptions in R. L. Lamberts, Application of Sine-Wave Techniques to Image-Forming Systems, J. SMPTE, 71 (9), 1962, J. C. Dainty et al., Image Science, Academic Press (1974), Chap. 7, and T. H. James et al., The Theory of the Photographic Process, 4th ed., Chap. 21. As the the spatial frequency, there is a description in the above The Theory of the Photographic Process, Chap. 21.
However, such an improvement in sharpness by reducing the thickness of the coated emulsion layers to heighten sharpness in the high spatial frequency region alone is insufficient since camera miniaturization has been rapid and enlargement magnification requirements for images is being raised.
Accordingly, methods for further improving sharpness in the high spatial frequency region without adverse effects have been investigated.